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[M16Ni24(CO)40]4-: Coinage Metal Tetrahedral Superatoms as Useful Building Blocks Related to Pyramidal Au20 cluster (M = Cu, Ag, Au). Electronic and Bonding Properties from Relativistic DFT Calculations Franck Gam, Ramiro Arratia-Pérez, Samia Kahlal, Jean-Yves Saillard, and Alvaro Muñoz-Castro J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00227 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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The Journal of Physical Chemistry

[M16Ni24(CO)40]4-: Coinage Metal Tetrahedral Superatoms as Useful Building Blocks Related to Pyramidal Au20 cluster (M = Cu, Ag, Au). Electronic and Bonding Properties from Relativistic DFT Calculations Franck Gama,b, Ramiro Arratia-Pereza,* Samia Kahlalb, Jean-Yves Saillardb* and Alvaro Muñoz-Castroc,* a

Doctorado en Fisicoquímica Molecular, Universidad Andres Bello, República 275, Santiago,

Chile b

UMR-CNRS, 6226 "Institut des Sciences Chimiques de Rennes" Université de Rennes 1,

35042 Rennes Cedex (France). c

Laboratorio de Química Inorgánica y Materiales Moleculares, Facultad de Ingenieria,

Universidad Autonoma de Chile, El Llano Subercaseaux 2801, Santiago, Chile

Abstract Characterization of the tetrahedral Au20 structure in the gas-phase remains as a major landmark in gold cluster chemistry, where further efforts to stabilize this bare 20-electron superatom in solution, to extend and understand its chemistry have failed so far. Here, we account for the structural, electronic and bonding properties of [M16Ni24(CO)40]4- (M = Cu, Ag, Au) observed in solution for gold and silver. Our results show a direct electronic relationship with Au20, owing that such species share a common tetrahedral [M16]4- central core with a 1S2 1P6 1D10 2S2 jellium configuration. In the case of Au20, the [Au16]4- core is capped by four Au+ ions, whereas in [M16Ni24(CO)40]4- it is capped by four Ni6(CO)10 units. In both cases, the capping entities are full part of the superatom entity where appears that the free (uncapped) [M16]4- species requires to be capped for further stabilization. It follows that the Ni6(CO)10 units in [M16Ni24(CO)40]4- should not be considered as external ligands as their bonding with the [M16]4- core is mainly associated with a delocalization of the 20 jellium electrons onto the Ni atoms. Thus, the [M16Ni24(CO)40]4- species can be seen as the solution version of tetrahedral M20 clusters, encouraging experimental efforts to further develop the chemistry of such complexes as M(111) finite surface section structures, with M=Ag and Au and, and particularly promising with M=Cu. Furthermore, optical properties were simulated to assist future experimental characterization. 1 ACS Paragon Plus Environment

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Introduction Since the beginning of this century, the chemistry of ligand-protected Group 11 superatoms1–3 has been tremendously developing, boosted in particular by the various potential applications of these atom- and electron-precise species.4,5 These clusters consist of a compact (generally pseudo-spherical) Mn core embedded in an outer protecting (“passivating”) shell made of various stabilizing ligands, such as for instance thiolates, halogenides and/or phosphines.2,5–8 In the specific case of thiolates, additional peripheral M(I) atoms are also present in the protecting shell, in which they occupy bridging positions between sulfur atoms. As a result, metal-thiolato “stapples” are formed, which are anchored to the Mn core through the coordination of sulfur.9–11 Owing to the presence of formally anionic ligands on the outer shell, the inner metallic core is generally in a positive oxidation state ([Mn]x+), giving rise to a non-integer averaged oxidation state [(n - x)/n)] lying between 0 and +1. The stability of superatoms is associated with “magic” electron numbers which provide them with a closed-shell configuration. These “magic” electron counts (2, 8, 18, 20, 34, 40, 58…), can be rationalized within the framework of a spherical jellium-type model leading to one-electron cluster orbitals somehow resembling the atomic orbitals, called superatomic orbitals,5,12,13 and ordering as 1S < 1P < 1D < 2S < 1F < 2P < 1G….1,6,12,14–17 Interestingly, the naked neutral [Au20] tetrahedral cluster, obtained in the gas phase and further characterized by photoelectron spectroscopy,18 exhibits a closed-shell superatom configuration. It is characterized by a 20-electron “magic” number (the 5d(Au) electrons are not included in this counting). Its electronic structure, investigated by density functional calculations,18–23 exhibits a large HOMO−LUMO energy gap, in agreement with its remarkable stability and unique optical and catalytic properties.18,24–30 Its structure is that of a bulk face-centered cubic (fcc) gold fragment, in a finite nanosized cluster motif.18,26 Efforts to bring [Au20] species into solution has been carried out since the last ten years,26,31–35 which resulted in structures and electronic structures completely different from that of the bare [Au20] cluster. Interestingly, in the beginning of the 90’s, Dahl’s group reported the stabilization of an octahedral Au6 core from the reaction between the Longoni–Chini [Ni3(CO)6]2- cluster36 as reducing agent of a Au(I) solution. The resulting [Au6Ni12(CO)24]2cluster37,38 was a first example of the capability of Group 11 cores to be embedded in an outer shell made of organometallic units, which can increase the versatility of the protecting layer. Later, the same approach was extended to achieve high-nuclearity counterparts employing 2 ACS Paragon Plus Environment

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Ag(OAc) and gold trichloride in modified conditions,39 resulting in the formation of [Ag16Ni24(CO)40]4- and [Au16Ni24(CO)40]4-, respectively, with the former characterized via single-crystal X-ray diffraction.39 Both solid-state structures of [Au6Ni12(CO)24]2- and [Ag16Ni24(CO)40]4- exhibit an ideal Td symmetry. At first sight, they can be viewed as made of a superatomic core, ([Au6]2- and [Ag16]4-, respectively) covered by four approximatively planar Ni3(CO)6 and Ni6(CO)10 units, respectively. However, a rationalization of the bonding between the superatom core and the nickel carbonyl units is not as straightforward as for thiolate- or phosphine-covered superatoms. Indeed, [:SR]- or :PR3 are two-electron ligands, making localized 2-electron/2center bonds with the metal core. In [Ag16Ni24(CO)40]4- for example, each Ni6(CO)10 units has several bonding contacts with the cluster core (Figure 1), suggesting the possibility for delocalized bonding. Moreover, it is likely that the electron-donation is going in the opposite way as in the case of classical ligands, i.e., from the core to the outer shell.40 Therefore, the question of the outer nickel carbonyl shell being not to be considered as a protecting ligand but as a full part of the superatom entity arises. In any case, the role of the nickel carbonyl units in the overall stabilization of these species is particularly puzzling. Although DFT investigations of [Ag16Ni24(CO)40]4- by Walter have recently appeared,13 they were not focused on the nature of the bonding between the nickel carbonyl groups and the Group 11 core, but to provide a analysis of the overall electronic structure. Herein, we are interested in the role of the nickel carbonyl entity in the stabilization of the whole cluster, extending the exploration along the Group 11 triad in the series [M16Ni24(CO)40]4- (M = Cu, Ag, Au). Both electronic and structural features are rationalized in terms of the superatom concept,6,10,13 showing a strong resemblance to the bare [Au20], thus, proving that the synthesis in solution of a related passivated core is possible, which can be useful for further explorations of its physico-chemical properties towards building blocks for nanostructured materials.41 On the other hand, this allows the possibility to access towards finite fcc fragments made of group 11 elements, for further understanding the size-dependent behavior and properties of a few atoms section of group 11 M(111) surfaces, which are widely employed as active supporting surfaces.42–46

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Figure 1. Molecular structure of [Au6Ni12(CO)24]2- (left) and [Ag16Ni24(CO)40]4- (right).

1. Computational details All density functional theory calculations were carried out by using the Amsterdam Density Functional (ADF) program47 and the zeroth-order regular approximation (ZORA) was applied to incorporate scalar relativistic effects in our calculations. The triple-ζ Slater basis set was employed, plus two polarization functions (STO-TZ2P) for valence electrons, within the generalized gradient approximation (GGA) according to the Becke-Perdew (BP86) exchange functional.48,49 The frozen core approximation was applied to the [1s2-4f14] inner electrons for Au, [1s2-4p6] for Ag, [1s2-3p6] for Cu, [1s2-3p6] for Ni, [1s2] for C and [1s2] for O, leaving the remaining electrons to be treated variationally. A gradient convergence criterion of 10−5 and an energy convergence criterion of 10−8 were utilized to perform our geometry optimizations. Analyses of the interaction energy between fragments constituting the investigated clusters have been carried out within the Morokuma-Ziegler energy decomposition method.50–52 Timedependent DFT (TD-DFT) calculations were employed at the same level, but using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional53 because offers a valuable comparison to the available computational calculation of UV-Vis spectrum for related clusters.4,54–57. It is worth mentioning that in the particular case of the investigated compounds, TD-DFT results at the BP86 level provided very similar results.

2. Results and discussions 2.1 The M16 core In [Ag16Ni24(CO)40]4-,39 the Td Ag16 core can be described in terms of two concentric shells, Ag’4@Ag”12, with two types of symmetry-equivalent metals. As said above, it can also 4 ACS Paragon Plus Environment

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be viewed as a compact piece taken out of an fcc metal. Indeed, along anyone of its four 3fold axes, the packing goes in three successive compact planes, as sketched in Figure 2: A (7 atoms), B (6 atoms) and C (3 atoms). Thus, the Ag16 core has four 7-atom and four 3-atom faces. In [Ag16Ni24(CO)40]4-, each of the four 7-atom faces is capped by a Ni6(CO)10 unit. Previous calculations by Walter13 lead to describe this cluster core as a 20-electron [Ag16]4superatomic subsystem with the spherical jellium configuration 1S2 1P6 1D10 2S2. At this point of the discussion, one should remind that in this type of clusters, only the 5s(Ag) electrons are considered participating to the bonding within the superatom core, and consequently to the jellium electron count. The fully occupied 4d(Ag) levels remain basically non-bonding and low-lying. Then the similarity between the [Ag16]4- subsystem and [Ag20] (or [Au20]) are striking. Indeed, the neutral unligated 20-electron [Ag20] (or [Au20])18,21,23 adopts a tetrahedral structure that can be generated from the [Ag16]4- subsystem by capping its four triangular faces with four Ag+. In these [M20] superatoms, the compact plane stacking goes as A (7 atoms), B (6 atoms), C (3 atoms) and A (1 atom). The alternative description of [M20] in terms of concentric shells, M’4@M”12@M”’4, reveals three types of symmetry-equivalent metals.22 We have optimized the Td-[M16]4- and Td-[M20] (M = Cu, Ag, Au) species assuming Td symmetry and found them to be closed-shell minima in these spatial configurations. Their major computed results are provided in Table S1. The relationship between their 20-electron superatom electron configurations is evidenced by the plots of the [Ag16]4- and [Ag20] KohnSham jellium orbitals (Figure S1, supplementary information).

Figure 2. Illustration of the fcc packing (A, B and C planes) in Ag20 (left) and the Ag16 core of [Ag16Ni24(CO)40]4- (middle) and with one Ni6 plane (right). The red, yellow and black colors correspond to the symmetry-equivalent Ag’, Ag” and Ag’’’ atoms, respectively, in Ag20 (Ag’4@Ag”12@Ag”’4) and the Ag16 core (Ag’4@Ag”12). The green color corresponds to Ni. 5 ACS Paragon Plus Environment


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A Morokuma-Ziegler decomposition of the bonding energy50–52 between the [M16]4core and its [M4]4+ envelope in the [M20] equilibrium geometry has been performed (see Computational Details) and its components are given in Table 1. For the three metals, the total bonding energy between the two fragments is negative (stabilizing), with similar values for M = Cu and Au, whereas that corresponding to Ag is weaker. This is in phase with the computed cohesion energy of [M20] (Table S1, supplementary information) and with previous calculations on bare Group 11 metal clusters.58 In fact, in such bare metal clusters Ag behaves always a bit differently than Cu and Ag, because the periodical trend is likely to be counterbalanced by the relativistic effects.58 Among the three components of the bonding energy, the Pauli repulsion is, by nature, positive. On the other hand, the two other components (electrostatic and orbital interactions) are stabilizing and more or less follow the same trend as the total bonding energy. The a1 and t2 components are dominating the orbital interaction energy. They result from the participation to the bonding of the valence (n + 1)s orbitals of M+, with some (n + 1)pσ contribution. Indeed, within the cluster Td symmetry, the four valence s-type AOs combine into a1 + t2 and interact strongly with their 2S (a1) and 1D (t2 component) counterparts on the [M16]4- fragment. As a consequence, the occupation after interaction of the M+ (n + 1)s orbital is substantial (~ 1.0 e, see Table 1), whereas that of its (n + 1)pσ AO remains lower than 0.1 e in the three clusters. The less important e and t1 contributions of the orbital interaction energy (Table 1), as well as the (n + 1)p and nd populations of the capping atoms, are the trace of some bonding interaction of the M+ vacant (n + 1)pπ combinations (e + t1 + t2) with occupied [M16]4- counterparts, as well as of the M+ nd occupied combinations (a1 + 2e +2t1 + 3t2) with some vacant [M16]4- MOs. In any case, the participation of the M+ (n + 1)p and nd AOs in the bonding with the [M16]4- core remains minor. Table 1. Decomposition of the bonding energy between the [M16]4- core and its outer capping [M4]4+ fragment in the [M20] (M = Cu, Ag, Au) clusters of Td symmetry and electron configuration of the four capping atoms. All energies are in eV. Compound (Td)

[Cu20]

[Ag20]

[Au20]

Fragmentation

[Cu16]4- + [Cu4]4+

[Ag16]4- + [Ag4]4+

[Au16]4- + [Au4]4+

Pauli repulsion

23.30

21.13

30.66

Electrostatic interaction Orbital interaction decomposition

-72.60 a1 a2 e t1

-64.79 -4.24 -0.01 -1.41 -1.38

a1 a2 e t1


-75.02 -3.87 -0.01 -1.00 -0.89

a1 a2 e t1

-5.63 -0.02 -1.23 -2.05

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t2 Total orbital interaction Total bonding energy Jellium electron configuration of the [M16]4- fragment Electron configuration of the outer capping atoms

-9.33

t2

-8.40

t2

-12.45

-16.38 -65.68

-14.17 -57.83

-21.38 -65.75

1S1.95 1P5.79 1D7.83 2S0.79 2P0.24 1F0.53

1S1.98 1P5.94 1D7.36 2S0.71 2P0.09 1F0.37

1S1.95 1P5.91 1D7.74 2S0.56 2P0.12 1F0.76

3d9.81 4s0.93 4p0.22

4d9.89 5s0.96 5p0.14

5d9.71 6s1.14 6p0.19



2.2 The Protecting Ni6(CO)10 units Being centered on a C3 axis of the tetrahedral [M16Ni24(CO)40]4- cluster (Figure 1), each of the four Ni6(CO)3(µ-CO)6(µ3-CO) units has local C3v symmetry. In fact, one can view the nickel atoms as part of the whole metal compact packing of the cluster (Figure 2, right side), which, along each of the C3 axis goes as C (M3), B (M6), A (M7) and B (Ni6). If, in a first step, we do not consider the central µ3-carbonyl ligand, the remaining Ni6(CO)9 fragment is almost planar, of idealized D3h symmetry (Scheme 1). This approximately planar Ni6(CO)9 neutral fragment has (6 x 10) + (9 x 2) = 78 electrons in the metal coordination environment. This is the electron count that one would predict at first sight, assuming a localized 2-electron/2center bonding scheme (nine 2-electron Ni-Ni bonds), with the six in-plane coordinated metals satisfying the 16-electron rule [(6 x 16) - (2 x 9) = 78]. However, it appears from the nearly planar 6-fold connectivity of the three inner Ni atoms (Scheme 1) that a localized bonding scheme with 9 Ni-Ni bonds does not apply properly. Indeed, the inner Ni atoms have only 5 in-plane valence orbitals (s, px, py, dx2-y2 and dxy) available for making 6 “bonds”. Considering that the metal dx2-y2 and dxy AOs are participating in 6 among the 12 Ni-(µ-CO) bonds (those involving the π*(CO) orbitals) and in all the Ni-Ni bonds, one is left with (6 x 2) – 6 = 6 localized 2-electron/2-center Ni-Ni bonds. A reasonable Lewis description of the Ni6(CO)9 fragment would be to discard the three central Ni-Ni bonds in Scheme 1. This would leave the three inner metals with a 14-electron configuration, whereas the three outer ones would remain 16-electron centers. This electron-deficient and delocalized situation makes this Ni6(CO)9 fragment somewhat different from structurally related species, such as [Pt3Fe3(CO)15]0/-/2-, for example.59–62 Adding now an out-of-plane µ3-CO on the central metal triangle of Ni6(CO)9 to complete the full Ni6(CO)10 organometallic unit reduces the electron deficiency of this triangle by 2 electrons and orientates the associated electron-accepting ability to the other side of the Ni6 plane.


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Scheme 1. Connectivity within a Ni6(CO)3(µ-CO)6 fragment in [Ag16Ni24(CO)40]4-.

-3

53e 40a1 -4

52e 51e

-5

39a1

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.04 eV

-6

16a2 38a1 -7

15a2

36a1 -8

50e 49e 37a1 48e 47e 46e

Ni6(CO)10 (C3v)

Figure 3. Kohn-Sham orbital diagram of free Ni6(CO)3(µ-CO)6(µ3-CO) (C3v symmetry).



The optimized geometry of the free Ni6(CO)10 unit (assuming C3v symmetry) and its Kohn-Sham MO diagram are shown in Figure 3. Its moderate out-of-planarity allows strengthening a bit the bonding around the metal centers by somewhat reducing their electron deficiency. The Ni-Ni bond lengths, 2.594 Å (inner) and 2.431 Å (peripheral) are consistent with the above qualitative description of the bonding. The electron deficiency is evidenced by the existence of low-lying vacant orbitals. In particular the 39a1 LUMO has significant inphase metal 4pz AO’s. The 16a2 HOMO has in-plane 3d-type character, whereas the 38a1 HOMO-1 is a 3dz2 combination, as well as the low-lying 36a1. Contrarily to the HOMO, these two last occupied MOs have favorable directional properties to interact with the [M16]4- core. 2.3 Interaction of a single Ni6(CO)10 with the [M16]4- core

The best way to analyze the role played by the outer organometallic shell in the bonding and stability of [M16(Ni24(CO)40)]4- is to first consider the interaction between one Ni6(CO)10 fragment and the [M16]4- core. In this respect, we have undertaken a Morokuma-Ziegler decomposition of the bonding energy50–52 between the [M16]4- and Ni6(CO)10 fragments in an [M16(Ni6(CO)10)]4- model of C3v symmetry, the structure of which (single-point calculation) being taken out of the optimized geometry of the [M16(Ni24(CO)40)]4- tetrahedral parent. Its components are given in Table 2. For the three models, the orbital interaction energy is dominated by its a1 and e components, in agreement with the fact that the directional properties of the Ni6(CO)10 orbitals of a2 symmetry are unfavorable. Moreover, an analysis of the fragment orbital populations after interaction indicates that the [M16]4- orbitals which are by far the most involved in the interaction are those which can be identified as the jelliumtype MOs (Figure S1). Among them, the 1D and 2S orbitals, which are occupied in the isolated [M16]4-, are dominating the interaction (see their population in the [M16(Ni6(CO)10)]4model in Table 2), whereas the 2P and 1F orbitals (lowest vacant levels in [M16]4-) interact in a lesser extent. Consistently, an examination of the occupation of the Ni6(CO)10 fragment after interaction (Table 2) indicates a major role played by the five lowest unoccupied organometallic MOs, in particular the 39a1 LUMO and the 51e LUMO+1 which are acting as strong accepting orbitals. The organometallic donor orbitals are principally of a1 symmetry (dz2 combinations) and much less efficient. Thus, the Ni6(CO)10 acts mainly as a strong electron acceptor, as exemplified by its largely negative charge in the computed model (Table 2). Moreover, its frontier orbitals interact mostly with the jellium-type orbitals of [M16]4- core. Clearly, this is not the behavior of a 2-electron ligand which would be expected to act as an 10 ACS Paragon Plus Environment

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electron donor and interact with core surface orbitals, different from the delocalized jelliumtype ones. Rather, the interaction of the Ni6(CO)10 fragment with the [M16]4- core resembles that of the four outer capping M+ atoms in the [M20] clusters (see above), despite of the fact they cap different faces of the [M16]4- core. Table 2. Decomposition of the bonding energy between the [M16]4- and Ni6(CO)10 fragment in the C3v model [M16(Ni6(CO)10)]4- (M = Cu, Ag, Au) and population analysis of the fragment. All energies are in eV. Compound (C3v)

[Cu16Ni6(CO)10]

Fragmentation

4-

[Ag16Ni6(CO)10]4-

[Au16Ni6(CO)10]4-

Pauli repulsion

[Cu16]4- + Ni6(CO)10 31.57

[Ag16]4- + Ni6(CO)10 24.90

[Au16]4- + Ni6(CO)10 26.17

Electrostatic interaction

-29.87

-23.24

-24.21

Orbital interaction decomposition

-5.34 -0.52 -7.90

a1 a2 e

-4.71 -0.42 -6.33

a1 a2 e

-4.43 -0.72 -6.69

a1 a2 e

Total orbital interaction

-13.77

-11.46

-11.85

Total bonding energy

-12.09

-9.79

-9.89

Jellium electron configuration 1S1.88 1P5.83 1D8.78 2S1.49 2P0.28 1S1.90 1P5.58 1D8.52 2S1.53 2P0.27 of the [M16]4- fragment 1F0.63 1F0.48

Occupation of selected Ni6(CO)10 frontier MOs

(36a1)1.90 (38a1)1.85 (16a2)1.92 (39a1)1.17 (51e)1.12 (52e)0.36

(36a1)1.87 (38a1)1.88 (16a2)1.92 (39a1)1.22 (51e)1.02 (52e)0.36

1S1.90 1P5.95 1D9.02 2S1.64 2P0.48 1F0.48 (36a1)1.88 (38a1)1.79 (16a2)1.86 (39a1)1.02 (51e)0.82 (52e)0.40

Ni6(CO)10 Mulliken charge

-1.64

-1.84

-1.43

2.4 The [M16(Ni24(CO)40)]4- clusters

The geometries of the [M16(Ni24(CO)40)]4- (M = Cu, Ag, Au) clusters have been fully optimized (see computational details). Major structural data are provided in Table 3, together with the computed HOMO-LUMO gaps which are consistent with their closed-shell superatom nature. The optimized distances of the silver species are in good agreement with the reported corresponding experimental values.39 Table 3. Relevant computed data for [M16Ni24(CO)40]4- (M = Cu, Ag, Au) clusters. ∆EH-L is HOMO-LUMO gap in eV. The two types of symmetry-equivalent group 11 atoms refer to the concentric spheres M’4@M”12 (see Section 3.1). The two types of symmetry equivalent Ni atoms are labeled NiA and NiB, the inner and outer metals in Ni6(CO)10, respectively (see Section 3.2). Distances are given in Å. Experimental values of Dahl’s silver cluster taken from Ref.39 are given in parenthesis. Td

[M’4@M"12@{Ni24(CO)40}]4-



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NiA-NiA

NiA-NiB

∆EH-L

M’-M’

M’-M"

M"-M"

M’-NiA

M"-NiA

M"-NiB

4-

1.06

2.625

2.493

2.481-2.503

2.681

2.652

2.558

2.703

2.446

[Ag16Ni24(CO)40]4-

0.83

2.995

2.865

2.832-2.897

2.838

2.973

2.731

2.703

2.449

(2.97)

(2.83)

(2.84)

(2.80)

(2.93)

(2.69)

(2.67)

(2.42)

4-

0.88

3.067

2.872

2.847-2.893

2.780

2.935

2.699

2.743

2.485

[Cu16Ni24(CO)40]

[Au16Ni24(CO)40]

Table 4. Decomposition of the bonding energy between the [M16]4- core its [Ni24(CO)40] envelope in the Td [M16(Ni24(CO)40)]4- (M = Cu, Ag, Au) and population analysis of the fragment. All energies are in eV. Compound (Td)

Fragmentation

[Cu16Ni24(CO)40]4-

[Ag16Ni24(CO)40]4-

[Au16Ni24(CO)40]4-

[Cu16]4- + [Ni6(CO)10]4 [Ag16]4- + [Ni6(CO)10]4 [Au16]4- + [Ni6(CO)10]4

Pauli repulsion

132.40

99.99

109.23

Electrostatic interaction

-125.13

-94.39

-102.49

Orbital interaction decomposition

a1 a2 e t1 t2

-6.52 -0.26 -8.51 -7.53 -25.62

a1 a2 e t1 t2

-5.22 -0.21 -6.84 -5.16 -22.36

a1 a2 e t1 t2

-4.65 -0.27 -6.35 -8.05 -21.94

Total orbital interaction

-48.44

-39.78

-41.26

Total bonding energy

-48.45

-34.18

-34.51

Ni6(CO)10 Mulliken charge

-1.68

-1.57

-1.12

The Morokuma-Ziegler bonding energy decomposition50–52 between the [M16]4- core and its complete [Ni24(CO)40] envelope is provided in Table 4. Both total bonding energy and its orbital component indicate stronger interaction with copper, whereas silver and gold behave similarly. The a1, e, t1 and (overall) t2 components of the orbital interaction energy are dominating. From the results obtained on the [M16(Ni6(CO)10)]4- model (see above), one can deduce that they result mainly from the participation of the pz and dz2 combinations of the individual [Ni6(CO)10] units (a1 and e in local C3v symmetry). From the jellium electron configuration of the [M16]4- and from the occupation of the frontier orbitals of a single Ni6(CO)10 fragment in the [M16(Ni24(CO)40)]4- cluster, it appears clearly that the major interactions between the [M16]4- core and its [Ni24(CO)40] envelope involves the occupied core jellium orbitals on one side and the accepting π-type orbitals (of substantial pz character) of the [Ni6(CO)10] unit on the other side. This result confirms that the behavior of the [Ni24(CO)40] envelope is not that of a “passivating” ligand shell, but rather of a full part of the superatomic entity. 12 ACS Paragon Plus Environment

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[Ag16Ni24(CO)40]4- (Td)

Figure 4. Kohn-Sham orbital diagram of [Ag16Ni24(CO)40]4-. The MO localization (in %) is given as follows: Ag16/Ni24/(CO)40. The Kohn-Sham MO diagram of [Ag16Ni24(CO)40]4- is shown in Figure 4. Those of the Cu and Au relatives (not shown) are similar. The highest occupied levels can be assimilated to the 2S (65a1) and 1D (81e and 145t2) jellium levels. The lowest unoccupied levels correspond to a mixture of the 1F and 2P jellium orbitals with π*(CO) combinations. 2.5 Optical properties

From the heterometallic nature of the title clusters, interesting optical properties may be anticipated. This is why a TD-DFT analysis has been undertaken in order to simulate their UV-vis spectra which are shown in Figure 5 allowing estimating characteristic patterns for further possible experimental realization. They exhibit similar features with four major absorption bands. The relevant associated electronic transitions are listed in Table 5 in the case of M = Ag. One can see that all the relevant transitions have a MLCT character 13 ACS Paragon Plus Environment


[M16(Ni24(CO)40)]4- (M = Cu, Ag, Au) clusters. The low-energy transitions are also associated with some core-ligand charge transfer character. The results, shows a sizable blue-shift of the main peaks for [Au16Ni24(CO)40]4- in relation to its copper and silver counterparts, revealing strong differences between the expected optical patterns along the series.

Figure 5. TD-DFT-simulated UV-vis absorption spectra of [M16Ni24(CO)40]4- (M=Cu, Ag and Au). Table 5. Major computed electronic absorption for [Ag16Ni24(CO)40]4-. The MO localizations (in %) are given in parentheses as follows: Ag16/Ni24/(CO)40. Photon Energy (eV)a 1.70 (0.13)

2.02 (0.11)

2.72 (0.09)

%

Major components of the transition

26

145t2 (23/60/17) —> 66a1 (24/37/39)

13

65a1 (59/21/20) —> 148t2 (17/29/54)

23

102t1 (5/77/18) —> 104t1 (23/35/42)

22

81e (32/48/20)—> 148t2 (17/29/54)

15

144t2 (3/50/47)—> 105t1 (0/33/67)

12

103t1 (6/70/24) —> 82e (0/36/64)

27

142t2 (8/83/9)—> 147t2 (15/29/55)

26

76e (4/86/10) —> 104t1 (23/35/42) 14

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3.23 (0.06)

a

21

134t2 (7/85/6)—> 146t2 (16/31/53)

18

142t2 (8/83/9) —> 83e (8/25/67)

13

97t1 (4/91/5) —> 82e (0/36/64)

Values in parenthesis correspond to oscillator strengths in a.u..

2.6 Bonding Considerations

So far, the closed-shell 20-electron [M16]4- superatom has not been reported, even as a ligand-passivated species, and one may wonder whether it is sufficiently stable for being isolated. Indeed, apart from its somewhat large negative charge, its 2S HOMO contains an antibonding interaction between the M4 inner core and its outer M12 cage, depicting a radial node. Despite its bonding nature within each individual M4 and M12 sphere, this orbital is likely lacking bonding character. A way to reinforce the 2S bonding is to add a third concentric sphere, made of 0-electron capping units having empty σ-type frontier orbitals, the in-phase combination of which being able to stabilize the 2S HOMO, whereas in addition the other combinations would also contribute to somewhat stabilize the 1D and 1P orbitals.

This is what 4-

in [M16Ni24(CO)40]

happens in [M20] (M’4@M”12@M4’’’),

but also

4-

([M’4@M”12@{Ni24(CO)40}] ). In the latter cluster, the four

Ni6(CO)10 fragment, which pack to the M16 core in a compact fashion, are mainly interacting with the occupied jellium orbitals by using their vacant σ-type frontier orbitals. This behavior is different from that of a 2-electron ligand (thiolate, halogenide, phosphine, etc…) which is expected to have its occupied frontier orbital to interact with a coreaccepting orbital localized on the superatom surface. Both situations are sketched in a schematic manner in Figure 6. Although very simplified, this description illustrates most of the differences between ligands and outer fragments belonging to the superatom core. The real situation is of course somehow more complex. In particular, significant mixing often occurs between the surface accepting orbitals of the superatomic core and the vacant antibonding jellium-type orbitals, which makes the distinction between them difficult. This is what happens for 1F and 2P MOs of the [M16]4- cores considered in this paper, which exhibit more accepting character than expected at first sight. Also, secondary bonding interactions occur between occupied MOs (mainly jellium-type) of the jellium core and vacant p-type of the supplementary superatom fragment.



Figure 6. Simplified MO interaction diagram illustrating the difference between an outer sphere atom belonging to the superatom entity (right side) and a (neutral or anionic) 2electron ligand (left side).

Conclusions In summary, our calculations indicate that [Ag16Ni24(CO)40]4- and [Au16Ni24(CO)40]4are strongly related to bare tetrahedral Ag20 and Au20 clusters, showing a 1S21P61D102S2 closed-shell configuration. In addition, the [Cu16Ni24(CO)40]4- counterparts is shown to be thermodynamically more stable than its Ag and Au relatives, thus, it should be possible to characterize it experimentally, similarly to its silver and gold counterparts. Hence, the insolution stabilization of the inner FCC-like M16 kernel motif as basic structure observed in the Au20 golden pyramid, allows to further explore the physico-chemical characteristics towards tetrahedral building blocks for nanostructured materials with novel properties. In addition, such structures represent a minimal model of four faces representing a (111) surface of a face-centered cubic unit cell in order to study catalitic activity, among other properties. We envisage versatile chemistry on the basis of the potential modifications in the surface of the M16 core.

ASSOCIATED CONTENT Supporting Information


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Relevant computed data for [M16]4- and [M20] clusters (M=Cu, Ag, Au) of jellium configuration 1S2 1P6 1D10 2S2, and Kohn-Sham jellium orbitals of [Au16]4- and [Au20]. Major electronic absorption for [M16Ni24(CO)40]4- (M = Cu, Ag, Au) computed at the PBE and BP86 levels. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Ramiro Arratia-Perez* [email protected] Jean-Yves Saillard* [email protected] Alvaro Muñoz-Castro* [email protected]

Author Contributions The manuscript was written through contributions of all authors. Aknowledgement The authors thank the financial support from FONDECYT 1140359, 1150629, and MILLENNIUM project RC120001 grants. The GENCI French national computer resource is acknowledged (grant a0010807367). This research has been performed as part of the ChileanFrench International Associated Laboratory for “Multifunctional Molecules and Materials” (LIA-CNRS N°1027). F.G. thanks the Région Bretagne (ARED NANOCLU) and the University Andres Bello for a joint PhD studentship.



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TOC graphics

A rationalization of Td [M16Ni24(CO)40]4- (M = Cu, Ag, Au), reveals strong analogy with the landmark Au20 cluster, on the basis of a common 20-ve [M16]4- core. Thus, further chemistry can be developed for a M(111) finite surface section of Group 11 elements.


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[M16Ni24(CO)40 ... - ACS Publications

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